Slush molding process

ABSTRACT

A process for slush molding hollow thin-walled flexible plastisol articles which begins with the application of an initial thin coating of liquid plastisol to the interior surface of a cooled open mold. This initial coating is then gelled by impinging pressurized hot gas directly onto the exterior surface of the mold, preferably while the mold is inverted and rocking about a horizontal axis. Thereafter, the thickness of this initially gelled coating is increased by applying additional liquid plastisol to the interior surface of the mold, which additional plastisol is then gelled by the latent heat stored in the mold wall during the previous heating step. The entire gelled plastisol coating is finally fused by a second impingement of pressurized hot gas directly onto both the exterior mold surface and the plastisol coating on the interior mold surface, while the mold is rotating about a horizontal axis.

United States Patent 1 Colby et al.

[ Apr. 17, 1973 SLUSH MOLDING PROCESS [75] Inventors: Daniel E. Colby,Somersworth; Philip E. Rogers, Dover; Frederick J. Sliwinski, Rochester,all of N H.

[73] Assignee: McCord Mich.

[221 Filed: Aug. 27, 1970 [21] Appl. No.: 67,631

Corporation, Detroit,

Related US. Application Data [63] Continuation of Ser. No. 756,178, July29, 1968 abandoned.

[52] US. Cl. ..264/302, 264/303, 264/310, 264/D1G. 6O [51] Int. Cl "B2905/12 [58] Field of Search 264/302-303, 236, 342, 310, 268, DIG. 60

[56] References Cited UNITED STATES PATENTS 3,002,230 10/1961 Stewart....264/244 2,588,571 3/1952 Porter ...264/302 2,939,180 6/1960 Hickleret al.-... ...264/302 3, l 34,831 5/1964 De Fusco et al .264/302PLASTISOL RESERVOIR II -lIlI 3,315,016 4/1967 Wersosky et a1. ..264/302Primary ExaminerRobert F. White Assistant Examiner-Gene AuvilleAttorney--Maurice E. Gauthier and McGlynn, Reising, Milton & Ethington[5 7 ABSTRACT A process for slush molding hollow thin-walled flexibleplastisol articles which begins with the application of an initial thincoating of liquid plastisol ,to the interior surface of a cooled openmold. This initial coating is then gelled by impinging pressurized hotgas directly onto the exterior surface of the mold, preferably while themold is inverted and rocking about a horizontal .axis. Thereafter, thethickness of this initially gelled coating is increased by applyingadditional liquid plastisol to the interior surface of the mold, whichadditional plastisol is then gelled by the latent heat stored in themold wall during the previous heating step. The entire gelled plastisolcoating is finally fused by a second impingement of pressurized hot gasdirectly onto both the exterior mold surface and the plastisol coatingon the interior mold surface,

while the mold is rotating about a horizontal axis.

9 Claims, 10 Drawing Figures PATENTED APR 1 H975 SHEET 1 [IF 3Izwezzioa'w.- Daniel E 60221 J Slizwizmidt,

PATENTEDAPRIYW 3728,1129

SHEET 2 0F 3 47 I I sea Daniel E. 002 3 3,

03121,?) 80932 15, 33 M M fl fliiofinegys PATENTEUAPRITIQB 3,728,429

' sum 3 OF 3 l FILL ll GELLATION AND ROCKING Ill REFILL WITH OWELL TIMEFOR ADDITIONAL GELLATION |v FUSION WITH. ROTATION V COOLING SLUSHMOLDING PROCESS CROSS-REFERENCES TO RELATED APPLICATIONS BACKGROUND OFTHE INVENTION This invention relates generally to the molding or Icasting of hollow articles from a flowable material which solidifieswhen subjected to heat, and more particularly to a process and apparatusfor slush molding plastisol skins for padded automobile components suchas arm rests, crash pads and the like.

A number of prior art slush molding operations have been developed inthe past. A typical example of one such method is disclosed in U.S. Pat.No. 2,588,571 (Porter). The Porter method includes the steps of applyinga coating to the interior surface of the mold by a process of fillingand dumping, and thereafter running the mold into a radiant heat ovenfor the purpose of gelling or preliminarily curing the initial coating.This step may be repeated again on selected portions of the mold:where athicker coating is desired. Thereafter, the. mold is placed in anotherradiant heat oven for the final cure.

Experience has indicated that a number of rather serious problems areassociated with this type of molding operation. To begin with, theprocedure of simply I filling and dumpingto coat the interior moldsurfaces prior to gellation may result inthin and/or bare spots in thefinished product. Frequently, this defect is caused by air bubbles whichbecome trapped between the liquidplastisol and the mold wall in moldrecesses and undercuts. The seriousness-of this problem naturallyincreases as parts take on more complex-designs. The

heating of molds in a large or openoven is the-source of still furtherproblems. For example, where complicated parts are being produced, openoven heating may fail to supply adequate heat to mold recesses andundercuts within the time alloted for gellation, the result being a skinwith inadequately gelled or cured-areas. Also, where the molds are heldin-a fixed inverted position during the gelling cycle, excess plastisolmay run down the mold walls and accumulate and/or remaintrapped incorners, undercuts and the like. Such'excess accuproperly mate each moldwith the induction heating coils will often produce rejects, either dueto underheating or scorching. Another problem with induction heatinglies in the fact that once the mold is surrounded by the induction coil,further relative movement between the mold and coil is not possibleuntil the gelling or fusing cycle has been completed. Where acomplicated part is being produced, mold immobility during heating mayresult in an inability to properly gel or fuse the plastisol coating indifficult areas such as remote mold recesses and undercuts. Theseriousness of this problem may in some cases entirely preclude thepossibility of molding certain articles on an induction heated slushmolding line.

SUMMARY OF THE INVENTION vention to be. hereinafter described in greaterdetail, a

mulations of plastisol are-frequently objectionable, particularly insituations where metal inserts must subsequently be fully inserted intothe corners and undercuts of the plastisol skin.

A different prior art slush molding operation involving inductionheating is disclosedin ,U.S. Pat.- No. 3,315,016 (Wersosky, et, al.).The induction heating method is a distinct improvement over open ovenheating, primarily because it is much faster and moreefficient. Hereagain, however, experience has indicated that other problems areassociated with induction heating. For example, because of theirbimetallic plated.

construction, the induction heating molds are more difficult andexpensive tomake. Also, the induction heating coils usually must bespecially designed to accommodate each particular mold This limits theflexibility of any one production line because a mold change will inmanyinstances involve a corresponding change in the heating coils;Furthermore, failure to, properlyarrangethepla'ted bimetallic layers ofthe mold or. to

plurality of open molds are carried by a chain conveyor through a seriesof-statio'ns at'which separate operations are sequentially performed oneach mold. Beginning at the first station, an empty'cooled mold isinitially filled with a plastic material, such as for exampleliquidplastisol. The filled mold is then carried to a second station whereitis inverted and the plastisol contained therein durripedinto'anunderlying reservoir. After dumping, the interior surface of themold remains coated with a thin layer of ungelled plastisol. At thispoint, it should be noted that someareas of the interior mold surfacemay not have' receivedan even coating clue to air. bubbles having beentrapped between the 1 plastisol and mold wall during the initial fillingoperation. Also,aexcess puddles of plastisol'may begin to accumulate inrecesses, undercuts and'other areas of the inverted mold. Therefore,to'insure an even distribution of the liquid plastisol over the entireinner surface of the mold, the inverted mold is then rocked about afixed axis. This rocking action encourages plastisol flow into recesseswhere air had previously been trapped, and also promotes plastisol flowout of recesses where an excess of liquid plastisol is accumulating.While the mold is rocking-in an inverted position, a heating 5 chamberoroven is moved into an'operative position surrounding ;the mold. Theoven, which is suitably dimensioned to accommodate-continued rocking ofthe inverted mold,'.is.zprovided with a series'of orifices whichdirectistreams of pressurized hot' gas directly onto the exterior moldsurface. Heating in this'manner, with the mold.in=motion relative tothe'streams of hot gas impingingon its exterior surface, continues untilthe liquid plastisol coating is gelled; Experience has indicated thatthe resulting gelled coating extremely uniform .and-zwithout bares'pots'or excess deposits in recesses'and undercuts. This is due to acombination of factors. By rocking the-mold during gellation, the liquidplastisol,which incidentally undergoes a marked drop in viscosity duringheating and prior to gellation, is

deprived of the opportunity' to accumulate and gel in mold recesses andundercuts. Also, because the initial,

liquid plastisolcoating is'extre'mely thin, usually on the order ofabout 15 to 25 mils, air bubbles trapped I between the plastisol andmold wall will expand and The mold then continues on to a fourthstation, where,

after the expiration of a predetermined dwell time, it is again invertedand its contents dumped into a second underlying reservoir. At thispoint, the latent heat stored in the mold wall during the previousgellation step has caused additional plastisol to gel on the interiormold surface, thus increasing the thickness of the initially gelledcoating. The inverted mold is again rocked about the same axis and asecond oven of similar construction to the first moved into an operativeposition surrounding the mold. Heating now continues, again by virtue ofhigh temperature streams of hot gas impinging directly onto the exteriormold surfaces until complete gellation of the entire coating is insured.Thereafter, the mold is rotated about the same axis as the impingementof hot gas continues. Rotation of the mold exposes both the exteriormold'surface and the plastisol coating on the interior mold surface todirect impingement of hot gas until the plastisol coating has beencompletely fused. This continued motion of the mold during fusion againinsures more uniform heat distribution to all parts of the mold, withoutan excess accumulation of plastisol in recesses and undercuts. Directimpingement of hot gas on the plastisol coating also markedly decreasesthe time required to complete fu The mold is then moved through acooling zone where it may be sprayed with an appropriate cooling medium.Following cooling, the skin is stripped from the mold cavity, and themold returned by the chain conveyor to the initial filling station.

Apart from the advantages mentioned above, the present inventionembodies another important feature which contributes substantially toits adaptability to high speed commercial operations. More particularly,the ovens employed to gel and fuse the plastisol coatings may bedimensioned to accommodate a wide range of mold sizes and designs. Thisfeature obviates the necessityfor changing ovens each time a new productis being runthrough the slush molding line and thus markedly increasesthe adaptability and flexibility of the equipment being employed.

These and other objects and advantages of the present invention willbecome more apparent as the description proceeds with the aid of theaccompanying drawings, wherein:

FIG. 1 is a schematic illustration of a preferred embodiment of anautomated slush molding line employed in the practice of the presentinvention;

FIG. 2.is a perspective-view of one of the vertically movable ovens,with portions broken away;

FIG. 3 is a sectional view taken along line 33 of FIG. 2 showing theoven lowered to the operative position in order to apply heat to arocking mold at station ll;

inches, a surface of approximately 1.52 square feet,

and a volume of about 144.4 cubic inches.

. Referring initially to FIG. 1, one embodiment of an automated slushmolding line constructed in accordance with the principles of thepresent invention is schematically shown as including a sprocket-drivenchain conveyor 10 suitably adapted to support a plurality of moldsindicated typically by the reference numetal 12. Conveyor 10 is arrangedto run horizontally through a series of stations (numbered l-VI in thedrawings). Beginning at station I, a given mold 12, which had previouslybeen cooled and stripped, is filled with a metered amount of liquidplastisol. At this stage in the operation, the mold surface temperatureis preferably between l00l50F. Liquid plastisol 13 at a temperature ofabout l20l 25F and of a composition similar to that disclosed in US.Pat. No. 3,315,016, but without the filler (Crystalline CaCO is pumpedby means of a pump 22 into the mold from a central reservoir 14 througha main manifold line 16, metering valve 18 and fill pipe 20.

After filling, the mold is'carried by conveyor 10 to station II, atwhich point the mold is inverted and its.

tively thin plastisol coating 28 (See FIG. 5B) of about I 20 milsthickness is applied 'to the interior mold surface. At this point, theinitial coating 28 is still in a liquidstate, thus exhibiting a tendencyto flow downwardly along the interior surfaces of the inverted mold. Toavoid a build-up of excess plastisol in mold undercuts and recesses, andto encourage flow of plastisol into other areas which had not beencoated due to air having been trapped between the plastisol and moldwall, the inverted mold is then rocked in both a clockwise andcounterclockwise direction about an axis 30 passing through itsgeometric center.

At this point, it should be understood. that the procedure of fillingand dumping is but one of a number of different methods which may beemployed to apply initial coating 28. As an alternative a lesser amountof plastisol might be deposited in an upright mold and the moldthereafter rocked to wash plastisol up onto the upper interior moldsurfaces. Thereafter,

tive position indicated in dotted lines at 32a. As can be best seen inFIGS. 2 and 3, oven 32 is comprised basi cally of a box-like containeropen at the bottom, with an insulated roof 34, side walls 36 and endplates 39. The interior of the oven is subdivided by a perforated metalbaffle member 38 into a plenum chamber 42 and a heating area 44. One ormore gas burners B are mounted on the roof 34 of oven 32. The burnersare supplied by lines 46 with a mixture of gas and air, and by lines 47with compressed air. The gas is ignited and the products of combustionfed directly into plenum chamber 42. When the oven 32 is lowered to theoperative positionindicated at 32a in FIG. 1, the inverted mold 12 islocated within the heating area, there being ample room between the moldand the interior surfaces of baffle member 38 to permit continuedrocking of the mold as diagrammatically indicated in FIG. 3.

As the mold continues to rock, the gaseous products of combustion,heated to a temperature of about 800900F, exit through perforations 48in baffle member 38 to impinge directly onto the exterior surfaces ofthe inverted mold 12 in the form of a plurality of high velocity jetsindicated diagrammatically in the drawings by arrows 50. Heating in thismanner continues for a period of -25 seconds until the initial plastisolcoating 28 on the interior mold surface has gelled. By continuouslyrocking the mold during heat- 2 ing, a more uniform impingement of thehot gases on the mold surface is achieved, with less chance of localizedscorching. As soon as gellation is completed, at which point the moldhas been heated to a surface temperature of about 250F, the oven 32 isreturned to its raised inoperative position. Thereafter, the mold 12 isrotated to the upright position and moved by conveyor 10 to station lll.

At station Ill, the mold 12 is refilled from manifold 16 through asecond metering valve 52 and depending fill pipe 54. The mold is thenmoved on to Station IV. The mold remains filled for a predetermineddwell time, say about 10 seconds, during which the latent heat stored inthe mold wall as a result of the previous heating step at station llgels additional plastisol 56 (See FlG. 5C). The net effect of thissecond gellation is to increase the thickness of the initially gelledlayer, thus producing a thicker coating 28 (See FlG. 5D) having a totalthickness of between 40-80 mils.

At the expiration of the dwell time, the mold 12 is again inverted andits ungelled contents dumped into tray 58. This ungelled plastisol isdrained from tray 58 back through pipe 60 to central plastisol reservoir14. Immediately upon inverting the mold 12 at station lV, rocking againcommences about axis 30. As rocking continues, a second oven 62 of thesame or similar construction as oven 32 is lowered to an operativeposition indicated at 62a. Products of combustion, again at atemperature of about 800900F, are fed into plenum chamber 42 from burnerB, the latter being supplied with fuel and air by lines 46' and 47'. Thegaseous products of combustion escape from the plenum chamber throughperforations in baffle plate 38' to impinge directly onto the exteriormold surfaces. Heating in this manner continues for a brief interval,until complete gellation of the plastisol coating 28' of theinteriormold surface is assured. During this period, any excess plastisol whichmight otherwise accumulate in mold recesses and undercuts will againdrain downwardly into tray 58. Thereafter (See FIG. 4), the mold isrotated a full 360 about axis 30, with the result that both the exteriormold surface and the gelled plastisol coating 28' on the interiorsurface is exposed to the direct impingement of high velocity streams ofhot gas. Direct impingement of hot gas on the plastisol 5 coating 28'enables fusion'to be completed in a relatively short time, usuallywithin -30 seconds. Because the hot gas enters the mold, the heating ofremote recesses and undercuts is greatly facilitated. Also, continuedmotion of the mold relative to the impinging streams of hot gas, firstby rocking and then by rotation, again produces a more uniformdistribution of heat, with much less chance of localized scorching.

As soon as the coating 28' is completely fused, the oven is returned toits raised inoperative position. At this point, the mold surfacetemperature is approximately 350F. The mold is then moved'on to StationV where it is cooled by any conventional means, such as 20 for exampleby water spray nozzles 64. The cooled mold is then moved on to stationVI, at which point the completely fused skin is stripped. The empty moldis then returned by conveyor 10 to Station I.

The advantages to be gained from the practice of the 5 foregoing processshould now be apparent to those skilled in the art. These advantagesinclude the initial application of a relatively thin plastisol coating28 on the interior of the mold. This thin coating enables air bubblestrapped between the plastisol and mold wall to expand and burst moreeasily during gellation, thus producing a smoother and more uniformsurface on the resulting skin. By inverting and rocking or rotating themold during initial gellation and subsequent fusion, ample opportunityis provided for the removal of excess plastisol from mold recesses andundercuts. Flow l -of plastisol into uncovered areas is also encouraged.Uniform heating of the plastisol skin is made possible [by subjectingthe moving mold to direct impingement I ;of hot gas on the exterior moldsurface during gellation, and on both the exterior and coated interiormold surfaces during fusion.

While the invention has been described with respect to an apparatuswhich includes a horizontal conveyor carrying molds through a series' ofsix consecutive stations, it is apparent that this represents only oneof the means which may be employed to practice the process describedabove. For example, instead of employing a conveyor, the molds may bemanually transported from one station to the next. The number ofstations at which different operations are performed may be varied.

Furthermore, it will be apparent that the ovens 32 and 62 may take anyof a wide variety of forms. It may also be advisable under certaincircumstances to use means other than the ovens 32a and 62 for impinginghot gases on the mold surfaces during gellation and fusion.

The above-described process and apparatus are therefore to be consideredin all respects as illustrative and not restrictive, the scope of theinvention being indicated by the appended claims, and all changes whichcome within the meaning and range of equivalency of said claims areintended to be embraced thereby.

Weclaim:

l. A process for molding thin-walled plastisol articles comprising:

a. applying a relatively thin liquid plastisol coating to the interiorsurface of an open mold;

l l l I b. inverting the mold and imparting motion to the inverted moldabout an axis while the plastisol coating remains in a liquid state, themotion imparted to the mold about said axis I being such as to promotedrainage of excess liquid plastisol from mold undercuts and recesses; l

. initially gelling said plastisol coating by impinging a flow of hotgas directly onto the exterior mold surface, while the mold remainsinverted and also while the mold remains in motion about said axisthereby to cause said exterior mold surface to be substantiallyuniformly exposed to the hot gas;

. increasing the thickness of said initially gelled .coating' byapplying additional liquid plastisol to the previously gelled plastisoland employing the latent heat developed in the mold w'all during theprior heating step to gel said additional liquid plastisol;

. fusing said gelled plastisol coating by direct impingement of apressurized flow of hot gas on the exterior mold surface, the saidfusion being accompanied by rotation of the mold about said axisrelative to said impinging gas such that said flow of hot gases alsoimpinges directly onto the gelled plastisol coating on the interior ofthe mold; f. cooling the mold following fusion; and, g. thereafterstripping the resulting thin-walled plastisol article from said mold. 2.A process for slush molding hollow thin-walled articles comprising thesteps of: v

a. applying a relatively thin liquid plastisol coating to .the interiorsurfaces of an open mold, the application of said coating beingaccomplished by at least partially filling the mold with plastisol andthereafter inverting the mold to drain any excess liquid plastisol fromthe mold; a

b. imparting a rocking motion to the inverted mold about a given axiswhile the plastisol coating remains in a liquid state, the motionimparted to i the mold about said axis being such as to promoteimpinging hot gas directly onto the exterior mold surfaces while themold remains inverted and in rocking motion about said axis;

(1. increasing the thickness of the previously gelled plastisol coatingby refilling the mold with liquid plastisol, allowing the latent heatstored in the mold wall as a result of-the previous heating step to geladditional plastisol, and thereafter emptying the mold of any remainingliquid plastisol;

. fusing the gelled plastisol coating by impinging hot gaseous productsof combustion onto both the exterior mold surfaces and the gelledplastisol coating, while the mold is in rotating motion; and,

thereafter removing the resulting hollow thinwalled molded article fromsaid mold.

3. The process claimed in claim 2 wherein the application of saidrelatively thin coating of liquid plastisol is accomplished while thetemperature of the interior mold surface remains within a range ofbetween 100-l50F.

4. The process as claimed in claim 2 wherein the thickness of saidrelatively thin coating is approximately 15 to 25 mils. 5. The processas claimed in claim 2 wherein the said hot gas constitutes the gaseousproducts of combustion, the temperature of the hot gaseous products ofcombustion impinging on the exterior surfaces of the mold being between800900F.

continued drainage of excess liquid plastisol from the mold whilepreventing the same from accumulatingin mold undercuts and recesses; c.gelling said relatively thin plastisol coating by 6. The process asclaimed in claim 5 further characterized by initial gellation beingaccomplished within 15 to 40 seconds.

7. The process as claimed in claim 6 wherein the additional gellationoccasioned by the latent heat in the mold wall continues for a dwellperiod of approximately 10 seconds.

8. The process as claimed in claim 5 further characterized by the fusionof said gelled plastisol coating being accomplished while the mold isrotating about said axis.

9. The process as claimed in claim 8 wherein the temperature of thegaseous products of combustion impinging on the exterior mold surfacesand the gelled plastisol coating is between 800900F.

2. A process for slush molding hollow thin-walled articles comprisingthe steps of: a. applying a relatively thin liquid plastisol coating tothe interior surfaces of an open mold, the application of said coatingbeing accomplished by at least partially filling the mold with plastisoland thereafter inverting the mold to drain any excess liquid plastisolfrom the mold; b. imparting a rocking motion to the inverted mold abouta given axis while the plastisol coating remains in a liquid state, themotion imparted to the mold about said axis being such as to promotecontinued dRainage of excess liquid plastisol from the mold whilepreventing the same from accumulating in mold undercuts and recesses; c.gelling said relatively thin plastisol coating by impinging hot gasdirectly onto the exterior mold surfaces while the mold remains invertedand in rocking motion about said axis; d. increasing the thickness ofthe previously gelled plastisol coating by refilling the mold withliquid plastisol, allowing the latent heat stored in the mold wall as aresult of the previous heating step to gel additional plastisol, andthereafter emptying the mold of any remaining liquid plastisol; e.fusing the gelled plastisol coating by impinging hot gaseous products ofcombustion onto both the exterior mold surfaces and the gelled plastisolcoating, while the mold is in rotating motion; and, f. thereafterremoving the resulting hollow thin-walled molded article from said mold.3. The process claimed in claim 2 wherein the application of saidrelatively thin coating of liquid plastisol is accomplished while thetemperature of the interior mold surface remains within a range ofbetween 100*-150*F.
 4. The process as claimed in claim 2 wherein thethickness of said relatively thin coating is approximately 15 to 25mils.
 5. The process as claimed in claim 2 wherein the said hot gasconstitutes the gaseous products of combustion, the temperature of thehot gaseous products of combustion impinging on the exterior surfaces ofthe mold being between 800*-900*F.
 6. The process as claimed in claim 5further characterized by initial gellation being accomplished within 15to 40 seconds.
 7. The process as claimed in claim 6 wherein theadditional gellation occasioned by the latent heat in the mold wallcontinues for a dwell period of approximately 10 seconds.
 8. The processas claimed in claim 5 further characterized by the fusion of said gelledplastisol coating being accomplished while the mold is rotating aboutsaid axis.
 9. The process as claimed in claim 8 wherein the temperatureof the gaseous products of combustion impinging on the exterior moldsurfaces and the gelled plastisol coating is between 800*-900*F.